A semiconductor substrate having a piezoelectric effect outputs an electrical signal in accordance with pressure applied in a spacing direction between both the front (top) and back surfaces of the semiconductor substrate, that is, in the thickness direction of the semiconductor substrate. A resistance variation of the semiconductor substrate that corresponds to distortion caused by application of pressure is used for measuring pressure.
JP-A-7-253364, JP-A-7-19981 and Japanese Patent No. 3,166,015 disclose pressure detecting devices that include a semiconductor substrate as a pressure detecting element. In these pressure detecting devices, a semiconductor substrate and a pressure transmitting member for transmitting pressure to the semiconductor substrate are accommodated in a metal housing. Furthermore, a detecting electrode is equipped to the front surface of the semiconductor substrate, a lead member is equipped to the outer periphery of the semiconductor substrate and the electrode and the lead member are connected to each other by wire bonding to receive a signal.
When such a pressure detecting device as described above is applied to a combustion pressure sensor for an engine, the accommodating portion of the pressure transmission member of the housing is inserted in a hole of the engine block, and the pressure in the combustion chamber is received by the pressure transmission member and transmitted to the semiconductor substrate to thereby detect the pressure.
Current design requirements necessitate engines that are compact and size and light in weight. Accordingly, the mount space of the pressure detecting device is also required to be reduced in size. Therefore, it is also required to reduce the diameter (size) of the pressure detecting device, that is, the diameter (size) of the housing.
However, the conventional pressure detecting device has a constraint that has prevented a reduction in size of the housing. More particularly, in the conventional pressure detecting device as described above, because the semiconductor substrate and the lead member are bonded to each other by wire bonding, the lead member is located on the outer periphery of the semiconductor substrate. Therefore, the wire bonding portion containing the lead member is larger in size than the semiconductor substrate, and the diameter of the housing is determined by the size of the wire bonding portion, which is largest among the parts accommodated in the housing. More fully, the minimum diameter of the housing is constrained by the size of the wire bonding portion. However, because the wire bonding portion is larger than the semiconductor substrate, a restriction is imposed on the reduction in size of the pressure detecting device.
With respect to this problem, the inventor has experimented with designing the pressure detecting device to include electrodes equipped on the front and back surfaces of the semiconductor substrate. Also, the semiconductor substrate was sandwiched by a part of the metal housing and lead members. Accordingly, no wire bonding was expected to be required for lead-out of the electrodes on both the front and back surfaces concerned and the size (diameter) of the housing could be reduced.
With this design, in order to establish electrical conductivity, the housing at the front surface side of the semiconductor substrate, that is, the housing at the pressure-applied side of the semiconductor substrate is required to be formed of metal. More particularly, if the entire housing is formed of metal, it would be simply expected that a semiconductor-substrate sandwiched type electrode lead-out construction can be adopted. However, if the entire housing is formed of metal, in the case of a combustion pressure sensor as described above or the like, one end of the housing is exposed to a high-temperature measurement environment in a combustion chamber or the like and the high heat of the combustion chamber or the like is transmitted through the housing to the semiconductor substrate connected to a part of the housing. Therefore, the semiconductor substrate is increased to a high temperature, which may cause an operation error and lead to operational difficulties.